So-called "fossil fuels" such as petroleum and coal, which are used as fuel for thermal power plants and industrial boilers, contain sulfur (S). Therefore, combustion of fossil fuels produces sulfur oxides (SO.sub.x) in the exhaust gas. When the temperature of the exhaust gas drops, SO.sub.x reacts with water in the gas to produce sulfuric acid, which is condensed on a material surface having a temperature lower than a dew-point, permitting occurrence of sulfuric acid dew-point corrosion. Similarly, in flue gas desulfurization equipment used in various industries, reduction of gas temperature causes sulfuric acid dew-point corrosion, if an SO.sub.x -containing gas flows in the equipment. Hereinafter in this specification, for the sake of simplicity, the SO.sub.x -containing gas is referred to as exhaust gas.
Because of the above-mentioned phenomenon, in heat exchangers and other equipment used for exhaust gas systems, the exhaust gas temperature has been maintained at 150.degree. C. or higher so that sulfuric acid does not form dew condensation on the material surface.
However, in view of the recent increase of energy demand, and also from the viewpoint of the effective use of energy, recycling of heat energy is desired to be as effective as possible. For example, attempts have been made to lower the exhaust gas temperature of a heat exchanger to a point lower than the dew-point of sulfuric acid. Thus, materials having resistance against sulfuric acid have been demanded.
Unless the exhaust gas temperature is maintained at 150.degree. C. or higher, an exhaust gas of a typical composition and having a temperature of about 140.degree. C. permits dew condensation of about 80% concentrated sulfuric acid on the material surface. For such environment, various so-called "low alloy steels" have been used as steel stocks for structural use. This is because low alloy steels have higher levels of resistance against a high-temperature and high-concentration sulfuric acid than do general-purpose stainless steels.
Boshoku Gijutsu (vol. 26 (1977), p. 731 to 740) describes that sulfuric acid corrosion accelerates in a temperature range of 20 to 60.degree. C. lower than a sulfuric acid dew-point. This is because that the amount of condensed sulfuric acid reaches a maximum in the above-described temperature range. For this reason, unless the exhaust gas is maintained at 150.degree. C. or higher, generally, resistance against corrosion is most required in a temperature range in the vicinity of 100.degree. C, where the concentration of sulfuric acid becomes about 70%. However, in this temperature range, to say nothing of general-purpose stainless steels, even low alloy steels cannot be used because of high corrosion.
Patent Application Laid-Open (Kokai) Nos. 56-93860, 2-170946, 4-346638 and 5-156410 disclose that specific corrosion resistance materials are usable for a sulfuric acid environment.
Patent Application Laid-Open (Kokai) No. 6-128699 discloses a highly alloyed austenitic stainless steel which has excellent corrosion resistance in an environment containing sulfate ion, halide ion and oxidizing metal ion simultaneously. Patent Application Laid-Open (Kokai) No. 64-21038 discloses an austenitic stainless steel which has excellent pitting corrosion resistance, crevice corrosion resistance, stress corrosion cracking resistance and acid resistance. And Patent Application Laid-Open (Kokai) No. 58-52463 discloses a stainless steel which exhibits corrosion resistance in an environment containing hydrogen sulfide, and moreover, has excellent mechanical properties.